@Pinecone - just follow the instructions I have you above. Click the right arrow button (just above Cancel in the lower right) on the Manage Series dialog a couple of times, and the value will be come 1.01 for the selected book. You can't type it into the "Start At" box because that is necessarily limited to integers.

On second thoughts (after a quick look at the code) I am unlikely to change it as it creates too many issues. There are too many permutations to deal with when it comes to attempting to preserve existing "depth" of series numbering.

Since it is a niche requirement, and you have the ability to apply the numbering you want using the arrow buttons that is as good as it is going to get for now.

If all you ever did with the plugin was assign series values by completely ignoring whatever existing values the series had, then yes it could do that.

However it does attempt to preserve whatever existing numbering strategy the series has. So if I have books in a series of 1, 1.5, 2 and being them up in the plugin, the default behaviour is to keep them as 1, 1.5, 2. Were you to type "2" as the start number, the books will become 2, 2.5, 3 - not 2, 3, 4.

That is only possible thanks to some complex logic which all goes to pot if the user were allowed to type 1.1 or 1.01. Or if the existing series numbering started at 0.5 for instance. Too much effort for the sake of a mouse click on your part